The present paper describes the influence of the chemical structure of two aminoalkoxysilanes: 3- aminopropyltriethoxysilane (APTS) and N-(3-(trimethoxysilyl)-propyl)-ethylenediamine (TSPEN) on the morphology of thin layer hybrid films with phosphotungstic acid (HPW), a Keggin heteropolyanion. X-ray photoelectron spectroscopy analyses indicated that both silane films showed protonated amine species interacting with the heteropolyanion by electrostatic forces as well as the presence of secondary carbamate anions. The hybrid films have different surface morphology according to atomic force microscopy analyses. The hybrid film with TSPEN forms flatter surfaces than the hybrid film with APTS. This effect is ascribed to higher flexibility and chelating ability of the TSPEN on adsorbed molecules. Ultrasonication effect on surface morphology of the hybrid film with APTS plays a fundamental role on surface roughness delivering enough energy to promote surface diffusion of the HPW heteropolyanions. This diffusion results in agglomerate formation, which corroborates with the assumption of electrostatic bonding between the HPW heteropolyanions and the protonated amine surface. These hybrid films could be used for electrochemical sensor design or to build photochromic and electrochromic multilayers.

1. Self-assembled hybrid films of phosphotungstic acid and aminoalkoxysilanes on
SiO2/Si surfaces
Adriano L. Souza a
, Lygia A. Marques b
, Marcos N. Eberlin b
, Pedro A.P. Nascente c
,
Paulo S.P. Herrmann Junior d
, Fabio L. Leite e
, Ubirajara P. Rodrigues-Filho a,
⁎
a
Universidade de São Paulo (USP), Instituto de Química de São Carlos, P.O. Box 780, 13560–970, São Carlos, São Paulo, Brazil
b
Universidade Estadual de Campinas (UNICAMP), Instituto de Química, Laboratório Thomson de Espectrometria de Massas, 13083-970, Campinas, São Paulo, Brazil
c
Universidade Federal de São Carlos (UFSCar), Departamento de Engenharia de Materiais, 13565-905, São Carlos, São Paulo, Brazil
d
Empresa Brasileira de Pesquisa Agropecuária, Embrapa Instrumentação Agropecuária, Laboratório Nacional de Nanotecnologia para o Agronegócio, 13560-970, P.O.Box 741, São Carlos,
São Paulo, Brazil
e
Universidade Federal de São Carlos (UFSCar), Campus de Sorocaba, P. O. Box 3031, 18052-780, Sorocaba, São Paulo, Brazil
a b s t r a c ta r t i c l e i n f o
Article history:
Received 20 October 2010
Received in revised form 21 December 2011
Accepted 23 December 2011
Available online 30 December 2011
Keywords:
3-aminopropyltriethoxysilane
N-(3-(trimethoxysilyl)-propyl)-
ethylenediamine
X-ray photoelectron spectroscopy
Atomic force microscopy
alkoxysilanes
Self-assembled films
Phosphotungstic acid
The present paper describes the influence of the chemical structure of two aminoalkoxysilanes: 3-
aminopropyltriethoxysilane (APTS) and N-(3-(trimethoxysilyl)-propyl)-ethylenediamine (TSPEN) on the
morphology of thin layer hybrid films with phosphotungstic acid (HPW), a Keggin heteropolyanion. X-ray
photoelectron spectroscopy analyses indicated that both silane films showed protonated amine species inter-
acting with the heteropolyanion by electrostatic forces as well as the presence of secondary carbamate an-
ions. The hybrid films have different surface morphology according to atomic force microscopy analyses.
The hybrid film with TSPEN forms flatter surfaces than the hybrid film with APTS. This effect is ascribed to
higher flexibility and chelating ability of the TSPEN on adsorbed molecules. Ultrasonication effect on surface
morphology of the hybrid film with APTS plays a fundamental role on surface roughness delivering enough
energy to promote surface diffusion of the HPW heteropolyanions. This diffusion results in agglomerate for-
mation, which corroborates with the assumption of electrostatic bonding between the HPW heteropolya-
nions and the protonated amine surface. These hybrid films could be used for electrochemical sensor
design or to build photochromic and electrochromic multilayers.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
The modification of surfaces by silylation producing self-
assembled monolayers (SAMs) [1] has contributed to applications in
several areas such as sensors and biosensors [2–5] as well as the im-
mobilization of metallic catalysts [6–8]. For these purposes, the si-
lanes usually consist of three surface reactive groups, with labile
alkoxide or chlorine, at one extremity and an organofunctional
group (for example, a nitrogenated Lewis base) at the other.
The hydrolysis reaction of the labile alkoxy or chloro groups re-
sults in silanol groups which can react with themselves through
condensation reaction leading to oligomers linked by bridges of si-
loxanes or react with the hydroxyl groups on the surface [9]. The ki-
netics of these hydrolysis reactions [10,11], control the silane film
morphology and adhesion. Nanostrands of oligo-aminosiloxanes
can be obtained by deposition of aged hydrolyzed solutions of 3-
aminopropyltriethoxysilane (APTS) on glass surface [12]. Also,
non-ordered aggregates or agglomerates, deriving from these reac-
tions, previously deposit on the surfaces resulting in random and
rough film morphologies [13,14]. Kornherr et al. [15], studying the
early steps of silane adsorption on polar surfaces, simulated the ad-
sorption of octyltrihydroxysilane, 3-aminopropyl-trihydroxysilane,
and 3-mercaptopropyltrihydroxysilane single molecules on ZnO
surface from isopropanol solution by molecular dynamics calcula-
tions. The results indicated that the more polar silane adsorbs on
the surface with the backbone parallel to the oxide surface, there-
fore maximizing the interaction between both polar groups (OH
and NH2), and the surface silanol sites (SiOH). For instance, the
other two silane molecules showed parallel and orthogonal adsorp-
tion, with higher stability for the orthogonal adsorption mode.
Fiorilli et al. [16] presented the same conclusion for the adsorption
of APTS on silicon substrates via vapor process. Wu et al. [17], inves-
tigating the adsorption of 4-amino-butyldimethylmethoxysilane
(ABS) and octadecyltrichlorosilane (OTS) from non polar solvents
on wet chemically oxidized silicon wafer surfaces, confirmed
the formation of a Langmuir monolayer for OTS and a three-
dimensional multilayer film up to 7.0 nm in thickness for ABS ad-
sorption. The adsorption of ABS is therefore very sluggish. Cao et
Thin Solid Films 520 (2012) 3574–3580
⁎ Corresponding author at: Universidade de São Paulo, Instituto de Química de São
Carlos, P.O. Box 780, 13560-970, São Carlos, Brazil. Tel./fax: +55 16 3373 9976.
E-mail address: uprf@iqsc.usp.br (U.P. Rodrigues-Filho).
0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.tsf.2011.12.069
Contents lists available at SciVerse ScienceDirect
Thin Solid Films
journal homepage: www.elsevier.com/locate/tsf

2. al. [18] investigated the use of supercritical CO2 as solvent for silyla-
tion reaction with alkylchlorosilanes on nanosized silica gel and sil-
icon wafer. They emphasized the role of surface water film on
silylation process in dried solvent and the formation of oligomeric spe-
cies on the surface of di- and tri- chlorosilanes resulting in multilayer
films in contrast to monolayer formation of mono-chlorosilanes.
Watson et al. [19] studied the influence of solvent polarity on the ad-
sorption of an ureido silane on E-glass fibers and the influence of the
polarity of the washing solvent on the physisorbed silane removal
and on the reorientation of chemisorbed silane on the surface. Allen
et al. [20] studied the adsorption of 3-aminopropyltrimethoxysilane
from dry and diluted bicyclohexyl solution by time-of-flight second-
ary ion mass spectrometry (ToF-SIMS) obtaining homogeneous thin
films with thickness of up to 2 monolayers. Their depth-profile ToF-
SIMS results in the H-
and O−
species that lead them to characterize
the presence of a water ad-layer on top of the aminosilane film.
These results are in agreement with our previous results obtained
by laser desorption ionization mass spectrometry for the 3-
imidazolylpropyltrimethoxysilane from diluted acetonitrile solu-
tion [21]. Recently, Ofir et al. [22] proposed a synthesis strategy
for fabricating a dense amine functionalized SAM on hydroxylated
surfaces using (11-Bromoundecyl)-trichlorosilane (BUTCS) and (3-
bromopropyl)-trichlorosilane (BPTCS) precursor layers. The tech-
nique has proven useful for obtaining an amine-terminated SAM
based on the BUTCS but fails for the BPTCS-derived layer. This difference
is ascribed to the less ordered and less dense layer formed by BPTCS,
thus making it less durable to the basic environment at the last step of
the reaction, resulting in its partial or complete removal. The results of
Ofir et al. [22] and those from Wu et al. [17] indicate therefore the better
ordering of the less polar silanes, alkyl- and bromo- silanes, and longer
chain silanes, octadecyl- and undecyl-silanes.
The formation of the self-assembled hybrid films is currently used
in applications such as electrochemical sensor [23], catalysis [24] and
electrocatalysis [25]. Some examples can be found in the literature
where coordination compounds [26,27] and polyoxoanions [28,29]
have been immobilized on organic layers for different purposes.
Keggin-type heteropolyanions are highly symmetric anionic
oxocluster units of c.a. 10 Å in diameter. They are basically composed
of molybdenum, tungsten, or vanadium atoms — called addenda
atoms, which are surrounded by six oxygen atoms in a C4v symmetry.
They are also assembled in trimers and tetramers sharing edges and
corners [30]. In the centre of the structure there are other elements
called heteroatoms, linked to four oxygens, in a Td symmetry. The
heteroatoms can be almost any transition metal and non-metal
atoms. These heteropolyoxometalates can be envisaged as molecular
mixed oxides or quantum dots of the parent mixed-oxide structures,
as the [PW12O40]3−
cluster (phosphotungstic acid, whose abbrevia-
tion is HPW), which has a diameter of 10.7 Å and is shown in Fig. 1.
The unique size and architecture of these compounds have made
them useful in several areas such as in materials science, catalysis,
and medicine [31].
In this study, self-assembled hybrid films of the HPW with ami-
noalkoxysilanes bearing one and two amine functions were studied.
The two aminoalkoxysilanes chosen were APTS and N-(3-(trimethox-
ysilyl)-propyl)-ethylenediamine (TSPEN) (Fig. 2), which were
adsorbed on the wet-chemically grown SiO2 layers over n-type Si
(100), named SiO2/Si. The hybrid film was obtained by further ad-
sorption of the HPW on the silane SAMs. The influence of the chain
lengths and the number of Brønsted basic amine functions of the si-
lanes on the morphology of the hybrid films is also discussed. These
systems formed herein could be used to develop electrochromic de-
vices and/or photochromic sensors since the polyoxotungstate under-
go multiple reversible reactions of electron transfer, preserving the
molecular structure [32].
The adsorption times of the silanes in the SiO2/Si surfaces were
optimized by the static contact angles (CAs) measurements. To verify
the possible hydrolysis or agglomeration of these silanes, the silane
solutions in chloroform were studied by electrospray ionization
mass spectrometry (ESI-MS). The self-assembled hybrid films were
characterized by X-ray photoelectron spectroscopy (XPS) and by
atomic force microscopy (AFM).
2. Materials and methods
2.1. Materials
Plates of 1 cm2
were cut from n-Si (100) wafers (Goodfellow Cam-
bridge Ltd.). The chemicals used in this study were APTS, TSPEN (GE
Silicones do Brasil and Acros), acetone (HPLC grade, Sigma-Aldrich),
chloroform (anhydrous, Sigma-Aldrich), HPW (Sigma-Aldrich), HF
(40%), H2SO4 (95–98%) and H2O2 (29%) from Synth. All reagents
were used as received. The resistivity of the deionized water was
18.2 MΩ cm−1
and the nitrogen gas was purchased from AGA.
2.2. Silicon wafer preparation
The silicon surface was previously treated in order to obtain
smooth and clean surfaces of SiO2/Si. The wafers were immersed in
10% (v/v) HF solution for 10s at room temperature, rinsed with plenty
of deionized water and then they were immersed in “piranha solu-
tion”, which consists of a mixture of H2SO4 and H2O2 (2:1-v/v) for
5 min. After that, the substrates were rinsed with copious amounts
of deionized water and dried under a nitrogen stream.
2.3. Preparation of hybrid films
The adsorption method of the layers was the same as that
reported in [21] referring to liquid-phase adsorption. In this method,
the SiO2/Si plates were immersed directly in 0.1% (v/v) APTS or TSPEN
chloroform solutions at 298 K. The samples were then washed ultra-
sonically with chloroform for 15 min in order to remove the physical-
ly adsorbed silane overlayer and then dried under a nitrogen stream
at room temperature. The immersion time was optimized by static
Fig. 1. Structure of the HPW in a ball and stick model.
Fig. 2. Chemical structures of the aminoalkoxysilanes. APTS (a) and TSPEN (b).
3575A.L. Souza et al. / Thin Solid Films 520 (2012) 3574–3580

3. contact angles measurements. The optimal adsorption time corre-
sponded to the maximum value of the water static contact angle
since the silylation process replace highly polar and hydrogen bond-
ing silanol groups by the less polar and weaker hydrogen bonding
group alkylamine.
The hybrid films were produced by dipping the silylated plates in
the 2.10−3
mol L−1
solution of the HPW in acetone; for each silane
the immersion time used was the same previously optimized by
water static contact angles measurements. Afterwards, the plates
were washed ultrasonically with acetone for 15 min in order to re-
move the physically adsorbed silanes and dried under a nitrogen
stream at room temperature.
2.4. Electrospray Ionization Mass Spectrometry (ESI-MS)
ESI-MS spectra were obtained using a Waters Micromass QTof hy-
brid quadrupole time-of-flight mass spectrometer operating at 7000
mass resolution and 5 ppm mass accuracy using typical analytical
conditions as described elsewhere [33].
2.5. Water Static Contact Angles (CAs) measurements
The water static contact angle were measured in a home-built go-
niometer composed of a digital CCD camera connected to a horizontal
optical microscope (MM Optics, São Carlos, Brazil) and interfaced to a
microcomputer. The water drop volumes were set at 10 μL.
2.6. X-Ray Photoelectron Spectroscopy (XPS)
The XPS measurements were performed using a spectrometer
VSW HA-100 with spherical analyzer and non-monochromatic Al
Kα radiation (1486.6 eV). The pressure in the chamber was in the
range of 10−6
Pa. The high-resolution spectra were measured with
constant analyzer pass energies of 44 eV, which produce a full
width at half-maximum line width of 1.6 eV for the Au(4f7/2) line.
The take-off angle used for spectra acquisition was 90°. The spectra
were charge-corrected and the chemical compositions of the films
were verified as previously described [21]. No evidence for photode-
composition of the samples due to X-ray irradiation was observed
in the time scale of the measurements.
All XPS peaks in insulating samples spectra were fitted according
to Leclercq and Pireaux's method, using Gaussian–Lorentzian peaks
implemented in the Winspec package kindly supplied by Professor
Pireaux [34].
2.7. Atomic Force Microscopy (AFM)
AFM images were taken in a Topometrix microscope, model Dis-
coverer TMX 2010, using silicon nitride tips with radius of 50 nm
and a cantilever (V shape) with spring constant of 0.09±0.02 N/m
(nominal value). The measurements were acquired at room tempera-
ture and relative humidity of 40%. All images were obtained in the
contact mode at a scan rate of 2 Hz. The root-mean-square roughness
(RRMS) and the agglomerates analysis were calculated using the Scan-
ning Probe Image Processor (SPIP TM
) software from Image Metrolo-
gy, Denmark.
Two usual roughness parameters can be extracted from the AFM
analysis. They are the Average Roughness (Ra) and the RRMS and are
defined by equations below:
Ra ¼
1
L
∫
L
0 F xð Þj jdx ð1Þ
RRMS ¼
1
L
∫
L
0
F xð Þ½ Š
2
dx
#1
2
ð2Þ
where F(x) is the profile height above datum and L is the sampling
length [35]. Ra is the arithmetic average of the absolute values of
the profile height deviations from the mean line recorded within
the evaluation length. Also known as Arithmetic Average or Center
Fig. 3. ESI-MS spectra for the solutions 0.1% (v/v) in chloroform of the aminoalkoxysi-
lanes. APTS (a) and TSPEN (b).
Table 1
Evolution of the CAs of the self-assembled films of the aminoalkoxysilanes on SiO2/Si
surfaces in function of the adsorption time.
Adsorption Time (min) CAs for APTS film (°) CAs for TSPEN film (°)
10 37.6±3.0 48.6±3.6
30 34.5±2.2 47.6±2.7
40 56.6±2.3 62.0±1.9
60 55.4±2.9 63.4±1.3
Fig. 4. Long scan XP-spectra of the self-assembled films on SiO2/Si surfaces. APTS (a)
and HPW/APTS (b).
3576 A.L. Souza et al. / Thin Solid Films 520 (2012) 3574–3580

4. Line Average, Ra is therefore the area between the roughness profile
and its mean line or the integral of the absolute value of the rough-
ness profile height over the evaluation length. RRMS is the root mean
square of the profile height deviations from the mean line, recorded
within the evaluation length. Ra and RRMS are both representations
of surface roughness, but each is calculated differently. Ra is calculat-
ed as the average roughness of microscopic peaks and valleys of a sur-
face and RRMS is calculated as the root mean square of microscopic
peaks and valleys of a surface.
The main difference in the two scales is that RRMS amplifies occa-
sional high or low readings, while Ra simply averages them. For a
given surface, therefore, the RRMS value will be higher than the Ra
value (by approximately 11%). Thus, RRMS is used to control very
fine surfaces in scientific measurements and statistical evaluations.
Ra is not a good discriminator for different types of surfaces (no dis-
tinction is made between peaks and valleys) and is not a good mea-
sure of sealed surfaces. In addition to this, Ra can takes less account
of the variations of the low frequencies [36–38]. For these reasons,
RRMS was used in this study.
3. Results and discussion
The surface morphology of thin films can be driven by solution
and surface chemistry. By solution chemistry effects we mean any ag-
glomeration phenomena or reaction that result in oligomer or macro-
molecule adsorption on surfaces. There is a hydrolytic tendency of
silanes in solution and formation of oligomeric and polymeric species,
therefore in this article we have studied the solution chemistry of
APTS and TSPEN to confirm that the morphology of our films could
Table 2
Binding energy values for the components in the N 1 s XP-spectra of the self-assembled
films on SiO2/Si surfaces.
Film –NH2 binding
energy (eV)
–[NHC(O)O]−
binding
energy (eV)
–NH3
+
binding
energy (eV)
APTS 399.9 401.6 402.8
HPW/APTS 398.8 400.5 402.5
TSPEN 399.4 400.5 402.0
HPW/TSPEN 398.9 400.8 402.7
Fig. 6. High resolution W4f XP-spectrum of the self-assembled HPW/APTS/SiO2/Si hy-
brid film.
Fig. 7. High resolution N1s XP-spectra of the self-assembled films on SiO2/Si surfaces.
TSPEN (a) and HPW/TSPEN (b).
Fig. 5. High resolution N1s XP-spectra of the self-assembled films on SiO2/Si surfaces.
APTS (a) and HPW/APTS (b).
3577A.L. Souza et al. / Thin Solid Films 520 (2012) 3574–3580

5. only be assigned to surface chemistry caused by reaction on surface
groups. Therefore in the next section we describe the solution behav-
ior of these silanes in our experimental conditions.
3.1. Solution chemistry
ESI-MS spectra of the silane solutions were obtained to verify ag-
glomeration, hydrolysis, and/or oligomer formation from silanes and
are displayed in Fig. 3(a) and (b) for the solutions of APTS and
TSPEN, respectively.
The ESI-MS spectrum of the APTS solution 0.1% (v/v) in chloro-
form is characterized mainly by the detection of the protonated mol-
ecule [M+H]+
of m/z 222, as shown in Fig. 3(a). The ion of m/z 176 is
ascribed to [M+H−EtOH]+
. The other ions of higher m/z, that is, of
m/z 277 and 323, are ascribed to [M+K+NH3]+
and [M+K+NH3 +
EtOH]+
, respectively. The ESI-MS spectrum of the TSPEN solution
0.1% (v/v) in chloroform, Fig. 3(b), is characterized mainly by the
detection of the two ions of m/z 223 and 237. The first is ascribed to
[M+H]+
, and the second to an homologue, that is, [M+CH2 +H]+
.
This is an important statement since we can now be sure that the ad-
sorption is only of single alkoxy silane molecules and not silanol mol-
ecules or oligomer siloxane molecules. Therefore, any agglomerate
formation on the surface is a surface driven process and is not related
to adsorption of agglomerates formed in solution.
3.2. Surface chemistry
3.2.1. CAs measurements
Table 1 shows the temporal water contact angle behavior of the
self-assembled films of aminosilanes on the SiO2/Si surfaces. It is as-
sumed that after 40 min the constant values denote the steady state
conditions. This assumption is likely to be correct since the achieve-
ment of the steady state would result in no more changes in the
chemical or morphological surface features and a constant surface en-
ergy. This result is similar to values of contact angles for surfaces
modified with amine groups [17]. The increase in contact angles
(a) (b)
(c) (d)
Fig. 9. AFM images of the self-assembled films on SiO2/Si surfaces in scale of 20×20 μm. APTS (a), TSPEN (b), HPW/APTS hybrid film (c) and HPW/TSPEN hybrid film (d).
Fig. 8. High resolution W4f XP-spectrum of the self-assembled HPW/TSPEN/SiO2/Si hy-
brid film.
3578 A.L. Souza et al. / Thin Solid Films 520 (2012) 3574–3580

6. with the adsorption time indicates that these silanes adsorb with the
amine group interacting with silanol groups on the surface as de-
scribed by Kornherr et al. [15] for silane adsorption on ZnO surface.
It turns out that the silanes would be adsorbed with non-
preferential absorption in the experimental conditions reported here-
in with a random orientation of the molecules in relation to the
surface.
3.2.2. Hybrid HPW–aminosilanes films
The most important features of the long-scan XP-spectra for the
immobilization of the HPW and aminosilanes on the SiO2/Si surfaces
are the presence of the N1s and W4f peaks, confirming the modifica-
tion of the surfaces with each aminosilane and the HPW. Fig. 4(a)
shows the long scan spectrum of the APTS/SiO2/Si film as a represen-
tative spectrum for both silane films. This spectrum is representative
since these silanes have the same features, confirming the adsorption
of the silanes. Fig. 4(b) shows the long scan spectrum for the HPW/
APTS/SiO2/Si hybrid film with the W4f peaks. These peaks confirm
the presence of the HPW. The spectrum for the film HPW/TSPEN/
SiO2/Si hybrid film showed the same features (not shown).
The high resolution N1s XP-spectra for the films APTS/SiO2/Si
and HPW/APTS/SiO2/Si are exhibited in Fig. 5. In each case there
was the appearance of an asymmetric peak, which was fitted with
three components assigned to amine groups (–NH2), carbamate
groups (–NHCOO−
) and protonated amine groups (–NH3
+
) [39,40].
Table 2 shows the binding energy values for these species. The pro-
tonation could result from the interaction between the APTS and the
surface silanol groups with a consecutive proton transfer [39], pro-
ton transfer from water ad-layer or vapor, or by proton transfer
from HPW to the amine groups. For the hybrid film, the last hypoth-
esis is more likely correct considering the superacidity of the phos-
photungstate. The carbamate groups result from the reaction
between the APTS and atmospheric CO2 [12]. More evidence of the
formation of carbamate groups can be obtained by the C1s XP-
spectra (not shown). They exhibited an asymmetric peak, which
was fitted with three components at 285.0, 286.7, and 289.2 eV
where the last component was assigned to the carbamate groups
[41,42]. Similar C1s XP-spectra were obtained for the TSPEN/SiO2/
Si film before and after the adsorption of the HPW (not shown).
The W4f XP-spectrum for the HPW/APTS/SiO2/Si hybrid film, Fig. 6,
shows an asymmetric doublet peak, which was fitted with two spin-
orbit doublets. The first species appears at 34.7 eV and the second at
36.4 eV. They were ascribed to oxidation states W5+
and W6+
, respec-
tively [40]. The appearance of the W5+
oxidation state is assigned to a
minor photo-reduction of the sample after X-ray exposure.
The N1s XP-spectra for the films TSPEN/SiO2/Si and HPW/TSPEN/
SiO2/Si are shown in Fig. 7. In each case, there was also the appear-
ance of an asymmetric peak, which was fitted with three components
assigned to the amine groups (–NH2), carbamate groups (NHCOO−
)
and protonated amine groups (NH3
+
). Table 2 shows the binding en-
ergy values for these species. The reasons for the appearance of
these species are analogous to the APTS/SiO2/Si film and HPW/APTS/
SiO2/Si hybrid film cases. Interestingly, the relative concentrations of
the ammonium and carbamate species in the HPW/TSPEN/SiO2/Si hy-
brid film are not the same as observed in the HPW/APTS/SiO2/Si hy-
brid film. This difference could be explained by the higher
nucleophilicity of TSPEN when compared to APTS, resulting in a
higher stability of carbamate groups formed with TSPEN.
The W4f XP-spectrum for the HPW/TSPEN/SiO2/Si hybrid film,
Fig. 8, shows an asymmetric spin orbit doublet peak, which was fitted
with two spin-orbit components. The first species presents a W 4f7/2
peak at 34.8 eV and the second species, at 36.5 eV. They were ascribed
to oxidation states W5+
and W6+
, respectively, as described before.
The AFM images of the self-assembled films are shown in Fig. 9.
When HPW is adsorbed onto each silane film, the films become rough-
er with the appearance of globular deposits (Fig. 9(c) and (d)), con-
trasting with the flatter silane films (Fig. 9(a) and (b)). The RRMS
values and the size of globules (D) are shown in Table 3. The formation
of large globules in the hybrid film with APTS (100 nm≤D≤400 nm)
were assigned to supramacromolecular agglomerates on the surface
formed by clustering or crystallization of several HPW on the silane
layer, as also evidenced in [43], increasing the RRMS. The flatter surface
of the hybrid film (RRMS =0.8 nm) with TSPEN reflects the possibility of
this silane to chelate the phosphotungstate preventing the formation of
agglomerates during the ultrasonication stage. It turns out that this
chelating mode results in better accommodation of the phosphotung-
state anion, [PW12O40]3−
, inside the silane film. Therefore, this better
incorporation achieved decreases the possibility of interaction among
the ion pairs.
In order to verify the role of the ultrasonication stage on the HPW
agglomeration on the silane film, an additional AFM study was con-
ducted with the HPW/APTS/SiO2/Si hybrid film, which was a rougher
(b)(a)
Fig. 10. AFM images of the self-assembled films on SiO2/Si surfaces in scale of 20×20 μm without the ultrasound stage. APTS (RRMS =0.06 nm) (a) and HPW/APTS hybrid film
(RRMS =1.2 nm) (b).
Table 3
RRMS values and Size of globules (D) of the self-assembled films on SiO2/Si surfaces
(20×20 μm).
Film RRMS (nm) D (nm)
APTS 0.9 #
HPW/APTS 5.3 100–400
TSPEN 0.6 #
HPW/TSPEN 0.8 40–60
3579A.L. Souza et al. / Thin Solid Films 520 (2012) 3574–3580

7. film. Fig. 10 shows AFM images of the APTS/SiO2/Si (Fig. 10(a)) and
HPW/APTS/SiO2/Si (Fig. 10(b)) films without the ultrasonication
stage. In this situation, the RRMS values were 0.06 nm and 1.2 nm for
the APTS/SiO2/Si and HPW/APTS/SiO2/Si films respectively, in images
of 20×20 μm.
Semi-quantitative information can be extracted from N/Si and W/
Si atomic ratios from XPS results for the APTS/SiO2/Si and HPW/APTS/
SiO2/Si films. For the films containing only APTS, the N/Si atomic ra-
tios values were 0.077 and 0.12, respectively with and without the
use of the ultrasound. The smaller value found for the film subjected
to the ultrasound can be attributed to the removal of physically
adsorbed molecules of APTS of the substrate.
A similar conclusion can be obtained when the W/Si atomic ratios
for the HPW/APTS/SiO2/Si hybrid films are analyzed. The W/Si ratios
found were 0.13 and 2.3, respectively with and without the use of
the ultrasound. Again, the lowest value of atomic ratio obtained for
the film subjected to the ultrasound can be related to removal of
HPW molecules from the film.
So, the clustering effect of the ultrasonication becomes evident
due to the great increase in the RRMS values for the films, as depicted
in Table 3. Indeed, this clustering effect on the ultrasound is quite ev-
ident due to the electrostatic nature of the [PW12O40]3−
/NH3
+
interac-
tion in the hybrid films.
4. Summary and conclusion
In this paper, we have evaluated the influence of a number of
amine groups of two aminoalkoxysilanes on the morphology of self-
assembled hybrid films with one Keggin-type heteropolyanion,
HPW on silicon wafers. For this evaluation, the adsorption times of
the aminosilanes were optimized by using the static contact angles
measurements. The hypothesis of formation of aggregates or agglom-
erates in solution due to hydrolysis and condensation reactions of
these aminosilanes was discarded by ESI-MS studies. The immobiliza-
tions of the molecules and the identification of the present species
were evidenced by XPS analyses. The AFM measurements showed
that the hybrid film with TSPEN is flatter than the hybrid film with
APTS due to the two amine groups in the TSPEN, which “chelate” to
HPW to better accommodate the [PW12O40]3−
inside the silane film.
Silane films are strongly attached to the surface by covalent siloxane
bonds, and HPW adsorbs through electrostatic forces on the surfaces,
enabling us to design the surface morphology by sonication. The re-
sults herein reported expand the understanding of both the solution
chemistry of aminosilanes and the surface chemistry of hybrid films.
They can also help for the development of sensors and the application
of these SAMs in catalysis and electrocatalysis or electrochromic
devices.
Acknowledgments
The authors acknowledge CNPq (Conselho Nacional de Desenvolvi-
mento Científico e Tecnológico), FAPESP (Fundação de Amparo à Pes-
quisa do Estado de São Paulo) for financial assistance and ALS thanks
the CNPq and CAPES (Coordenação de Aperfeiçoamento de Pessoal de
Nível Superior) for the MSc and PhD fellowships respectively. The au-
thors are grateful to Prof. Richard Landers (IFGW/UNICAMP) and Rita
C. G. Vinhas for assistance with the XPS measurements.
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